1. Field of the Invention
The present invention relates generally to a method of manufacturing a Group III nitride substrate (a substrate including Group III nitride semiconductor crystals), and a semiconductor device.
2. Related Background Art
A Group III nitride compound semiconductor such as, for instance, gallium nitride (GaN) (hereinafter also referred to as a “Group III nitride semiconductor” or a “GaN-based semiconductor”) has been gaining attention as a material for semiconductor elements that emit blue or ultraviolet light. A laser diode (LD) that emits blue light is used for high-density optical disk devices or displays while a light emitting diode (LED) that emits blue light is used for displays, lighting, etc. It is expected to use an ultraviolet LD in the field of biotechnology or the like and an ultraviolet LED as an ultraviolet source for a fluorescent lamp.
Generally, substrates of a Group III nitride semiconductor (for example, GaN) that are used for LDs or LEDs are formed through vapor phase epitaxy. For instance, they are formed through heteroepitaxial growth of Group III nitride crystals on a sapphire substrate. However, the sapphire substrate and the GaN crystals are different from each other in lattice constant by 13.8% and in coefficient of linear expansion by 25.8%. Hence, a sufficiently high crystallinity cannot be obtained in the GaN thin film obtained through the vapor phase epitaxy. Generally, crystals obtained by this method have a dislocation density of 108 cm−2 to 109 cm−2 and thus the reduction in dislocation density has been an important issue. In order to resolve this issue, efforts have been made to reduce the dislocation density and thereby, for example, an epitaxial lateral overgrowth (ELOG) method has been developed. With this method, the dislocation density can be reduced to around 105 cm−2 to 106 cm−2, but the manufacturing process is complicated.
On the other hand, besides the vapor phase epitaxy, a method of carrying out crystal growth from a liquid phase also has been studied. However, since the equilibrium vapor pressure of nitrogen is at least 10000 atm (10000×1.013×105 Pa) at the melting point of Group III nitride single crystals such as, for instance, GaN or AlN, conventionally it has been understood that a condition of 8000 atm (8000×1.013×105 Pa) at 1200° C. is required for growing GaN from a liquid phase. In this connection, recently, it was made clear that GaN was able to be synthesized at relatively low temperature and pressure, specifically, 750° C. and 50 atm (50×1.013×105 Pa), by using a Na flux.
Recently, single crystals with the maximum crystal size of about 1.2 mm are obtained by a method in which a mixture of Ga and Na is melted in a nitrogen gas atmosphere containing ammonia at 800° C. and 50 atm (50×1.013×105 Pa), and then single crystals are grown for 96 hours using the melt (see, for instance, JP2002-293696A).
Furthermore, another method has been reported in which, after a GaN crystal layer is formed on a sapphire substrate through metalorganic chemical vapor deposition (MOCVD), single crystals are grown through liquid phase epitaxy (LPE).
Generally, for instance, a sapphire substrate is used for manufacturing a Group III nitride substrate. However, such a substrate and a Group III nitride crystal are different from each other in lattice constant and coefficient of thermal expansion. Hence, when Group III nitride crystals are grown using such a substrate, the substrate may be distorted or warped in some cases.
When a device is to be manufactured using a semiconductor substrate with low surface flatness, the manufacture may be difficult. For instance, mask alignment may be difficult, for example in the case of using a stepper to be employed in a device manufacturing process.